I would recommend going with a disc brake with drop spindle kit. upgrade to 73-87 balljoints and tie rods. then throw in drop springs while youre at it. check out http://www.earlyclassic.com/tech.aspx?category=Stance%20Photos-2

for stance photos but look around for cheaper kits. not sure about rear suspension on c20s but whether leaf or coil there are many easy options to lower it.